dt-bindings: reset: add bindings for the Meson-A1 SoC Reset Controller

Add DT bindings for the Meson-A1 SoC Reset Controller include file,
and also slightly update documentation.

Signed-off-by: Xingyu Chen <xingyu.chen@amlogic.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>